Next time you look up, give a wave
Exploring stars with their own planetary systems, from your back garden or bedroom window.
In 1995, using observations from an observatory in Haute-Provence, two astronomers spotted the tell-tale signs of a planet orbiting an ordinary star other than the Sun (Three years prior, detection of a planetary companion to a star had been made but in the extreme environment of a pulsar, which would be hostile to any opportunity for life and may well have formed as a secondary result of the merger of two white dwarf stars and hence be very different to our Solar System!).
The star, 51 Pegasi, can just about be seen with the naked eye (in the right conditions) and lies at a distance of 50.45 light-years. Although it was long expected that other stars would have planetary systems of their own (After all, why would the Sun be any different to the billions of other stars in the galaxy?), detecting strong evidence for them, was an extremely difficult challenge, requiring meticulous observation and high quality instrumentation.
The stars are so distant from us, that they appear as unresolvable points of light, brilliant enough too, to swamp any attempt at direct observations of nearby faint objects such as planets. Some have been detected by blocking out the star light and using sophisticated image processing. However, in the absence of a direct image, we can detect planets using two principal methods. One, is from the little dips in the light coming from the star as the planet passes between us and the star; little partial eclipses in each orbit. This, of course, only works for those planets whose orbits are aligned in the right plane, but given that there are so many stars, we should be able to see many such eclipses.
The second method is based on the fact that gravity works on all objects with mass, so that not only does the Sun (for example) attract each of the planets, but that each also attracts the Sun! In other words, the Sun and the planets orbit around their common ‘centre of mass’. Because the Sun is so massive compared to the planets, then it dominates the system and the centre of mass lies very close to the Sun (the main effect that pulls that central point out from just inside the solar atmosphere is the mass of Jupiter). This means that we can detect a ‘wobble’ in the position of the star. Measuring such tiny differences in position though is enormously difficult.
But, as such a ‘wobbling’ star moves away from us and then back towards us again, then the light from that star (if we look at the spectrum where we can see lines caused by different elements) will be red-shifted (moving away from us) and then blue-shifted (coming towards us). This Doppler shift means that we can effectively see the lines in the spectrum moving back and forth. This is called the radial velocity method.
OK, so where do we look in the night sky to see some of these stars with known planets?
Well, we’re not going to see the planets themselves, but at least we can look towards their parent star and just wonder on whether or not there is life over in that direction. Worthy of a little friendly wave perhaps?
Don’t worry if you’re not familiar with the constellations and the names of the brightest stars (we’ll come to those in another article!), there are plenty of apps you can get for your phone, or web-sites that can help you get oriented. The good news is that some of the parent stars are bright enough to be seen without a telescope or binoculars and the brightest of them is a well-known star, Aldebaran. That’s a good one to start with, because it is near Orion, which most people can recognise Orion from his three-starred ‘belt’, and indeed lies between the belt and the Pleiades (Seven Sisters) star cluster. It is the ‘eye’ of the bull, Taurus. See if you can find it! It’s actually a double star, with a large planet detected. If you were to send a greeting towards it, it would take 66 years for the signal to get there, and another 66 for the return call. Oh, and the Pioneer 10 space probe will actually pass by Aldebaran in about 2 million years from now.
Another easy to find one is associated with the constellation Gemini. The two main ‘twins’ are the bright stars Castor and Pollux, and we now know that at least Pollux has a planetary system. In this case, it is closer to us, at only 34 light-years. Again, we can use Orion to help us, and we just need to look over to the left and up a little to see these fairly obvious two stars, Pollux being the lower of the two.
The nearest exoplanet, however, is orbiting around the nearest star to us beyond the Sun! So it is only a little over 4 light-years away, and is known as Proxima Centauri b, and as a bonus, another planet has also been detected there, Proxima Centauri c. Unfortunately, for those of us in the Northern hemisphere, Centaurus is really a southern sky constellation and so I can’t point it out to you from here on my doorstep in Ireland!
For more stars with known planets, some of which are also easy to find, have a look at the list (and sky charts) drawn up by Sky & Telescope magazine. We’ve now detected well over 4,000 exoplanets and so the chances are very high that many, if not most, of the stars you see twinkling on these dark, cold winter nights have their own family of planets, of all sizes. Some of them also lie at just the right distance from their parent star for their surface temperature to be in the right range for life to form. So maybe that wave is worthwhile after all?
Ten bright stars you can see with the naked eye that have exoplanets https://www.forbes.com/sites/jamiecartereurope/2019/10/24/10-bright-stars-you-can-see-after-dark-tonight-with-your-naked-eye-that-nasa-says-host-exoplanets/?sh=50a63b6670a5
List of Exoplanets https://en.wikipedia.org/wiki/Lists_of_exoplanets
NASA’s exoplanet archive (access to range of data). https://exoplanetarchive.ipac.caltech.edu/
Free software for your computer to plot the night sky: Stellarium
As for apps? There are lots. Try Sky Safari and Google Sky. Many apps will also use your phone to work out which direction you are looking at and help you identify and stars you see.
